1. Field of the Invention
The present invention relates to semiconductor lasers and, more particularly, to semiconductor lasers having a quantum well structure which can be used for fiber optic communication, information processing utilizing light, and the like.
2. Description of the Related Art
Semiconductor laser devices having a quantum well structure of compound semiconductor material are, based on theoretical studies, expected to have excellent characteristics of performance such as a low threshold value, high conversion efficiency and small temperature dependency. Recently, intensive efforts have been made for research and development of these devices.
A quantum well semiconductor laser device is generally known to have the following structural features. A quantum well structure is interposed between a pair of guide layers and serves as a resonator of the laser device. The quantum well structure includes at least one quantum well layer and at least one barrier layer. The thickness of the quantum well layer is set sufficiently thin to generate quantum effect for electrons injected into the quantum well structure.
The above-mentioned advantages of quantum well semiconductor laser devices are based on the following theory. That is, because such a device allows diffusion of electrons only in directions in two dimensions and thus forms quantum levels within the quantum well layer, changes in density of states of electrons would be more abrupt and gain distribution generated by electrons would be more restricted than in the case of semiconductor laser devices having conventional double heterostructures wherein electrons diffuse in directions in three dimensions. On the other hand, because holes have heavier mass and thus smaller energy differences among existing quantum levels compared with the case of electrons, quantum effect is usually not significantly attained for holes within the quantum well structure. Therefore, characteristics of a quantum well semiconductor laser device are mainly affected by density of states of electrons.
The above-mentioned abrupt changes in density of states of electrons are known to be shown by electrons in all quantum levels, regardless of the number of quantum well layers and the length of the resonator of a semiconductor laser device. For this reason, the number of quantum levels allowed to exist within the quantum well structure, as well as the number of quantum well layers and the length of the resonator, have attracted little or no specific attention in studies for manufacturing quantum well semiconductor laser devices.
However, conventional quantum well semiconductor laser devices have not shown as good characteristics as have been expected. For example, semiconductor laser devices for household machines, such as those used for fiber optic communication between a telephone central office and the homes of subscribers, are often used under rigorous conditions and thus especially require satisfactory performance characteristics at high temperature. Nonetheless, conventional quantum well semiconductor laser devices to date have not satisfactorily realized desired characteristics such as those enabling laser oscillation at significantly high temperature while preventing saturation of optical output. This particular problem is believed to be caused because of the following reason. That is, as temperature of the semiconductor laser device rises and a threshold current density required for attaining laser oscillation increases, carriers injected into a quantum well layer overflow to an adjacent barrier layer, thereby further accelerating increase in the threshold current density.
To overcome the above-mentioned problem, attempts have been made to coat the faces of the resonator of a quantum well semiconductor laser device with dielectric layers having high reflectivity. Such coating can reduce the threshold current density and thus enables laser oscillation at higher temperature. However, these semiconductor laser devices are difficult to use for generating optical output of significantly high intensity.
Further, when used as a component in a fiber optic communication system, a semiconductor laser device is usually coupled with an optical fiber through a lens interposed therebetween. In order to improve transmission characteristics of the system, the semiconductor laser device should have high coupling efficiency with the optical fiber. It is known that achievement of higher coupling efficiency makes it desirable for laser light emitted from the semiconductor laser device to have a circular image rather than an elliptic image along a plane parallel to the light-emitting face of the device. Therefore, this desired characteristic would preferably be incorporated into structural features of quantum well semiconductor laser devices.